Solder Properties

Solders are alloys that have melting temperatures below 300°C, formed from elements such as tin, lead, antimony, bismuth, and cadmium. Tin—lead solders are commonly used for electronic appHcations, showing traces of other elements that can tailor the solder properties for specific appHcations. 

The second premise of process control is the repeatability of making an acceptable solder joint on each assembled circuit board. With the exception of a major equipment failure that alters the time-temperature profile, the capability to solder the same joint repeatedly in an acceptable manner is more a function of the consistent solderability of the material set s properties. Those solderability properties include the efficacy of the flux, the properties of the solder metal (e.g., surface oxidation and particular to the case of solder paste, the solder powder particle size, and metal loading), and the solderabihty of the component I/O and associated circuit board feature. Moreover, the solderability of component I/Os and circuit board features is often identified as the primary factor in repeatedly making acceptable interconnections. 

The two remaining sections describe the fundamental fatigue and creep properties of the Pb-free solders. The properties of the bulk solder, as well as those of solder joints, are highlighted. The relevance of bulk solder properties stems from the need to develop constitutive models that predict the long-term reliability of the solder interconnections. Each section will summarize quantitative data, as well as address microstructural considerations pertaiiung to deformation in Pb-free solder compositions. 

J. Clech, Review and Analysis of Pb-Free Solder Properties, Report to the NEMI Pb-Free Solder Project, 2003... 

T. Siewert, S. Liu, D.R. Smith, and J.C. Madeni, Database for Solder Properties with Emphasis on New Lead-Free Solders, National Institute of Standards and Technology Colorado School of Mines, 2002... 

A more detailed compilation of these properties and other solder properties can be found in the third edition of the Soldering Handbook (American Welding Society, 2000). 

Materials Reliability Division in the Materials Science and Engineering Laboratory of the National Institute of Standards and Technology, Database for solder properties with emphasis on new lead-free solders, release 4.0. (http //www.boulder.nist.gov/div853/lead%20free/solders.html). 

The ETEE copolymer can be cross-linked by radiation (30), despite the high content of tetrafluoroethylene units. Cross-linking reduces plasticity but enhances high temperature properties and nondrip performance. The irradiated resia withstands a 400°C solder iron for 10 min without noticeable effect. 

The solder and ahoy market, including low melting or fusible ahoys, is a principal user of indium (see SoLDERS AND BRAZING ALLOYS). The addition of indium results in unique properties of solders such as improved corrosion and fatigue resistance, increased hardness, and compatibhity with gold substrates. To fachitate use in various appHcations, indium and its ahoys can be easily fabricated into wine, ribbon, foil, spheres, preforms, solder paste, and powder. 

Engineering rework is possible with eutectic and solder materials, but impossible with silver—glass. This constraint severely limits the usefulness of the material. Tables 4 and 5 give the electrical, mechanical, and thermal properties for various adhesives. 

Possible tape materials include polyimide, polyester, polyethersulfone (PES), and polyparabanic acid (PPA) (18). Of these, polyimide is the most widely used material because its high melting point allows it to survive at temperatures up to 365°C. Although polyester is much cheaper than other materials, its use is limited to temperatures less than 160°C. PES and PPA, on the other hand, are half as cosdy as polyimide, and can survive maximum short-term temperatures of 220 and 275°C, respectively. PES has better dimensional stabiUty than polyimide, absorbs less moisture, and does not tear as easily however, it is inflammable and can be attacked by certain common solvents. Table 7 Hsts various plastic tapes and their properties. Common bump materials are gold, copper, and 95% Pb/5% Sn solder (see Tables 6 and 8 for properties see also References 2 and 21). 

Table 8. Mechanical and Electrical Properties of Solder Materials ...

Alloy selection depends on several factors, including electrical properties, alloy melting range, wetting characteristics, resistance to oxidation, mechanical and thermomechanical properties, formation of intermetaUics, and ionic migration characteristics (26). These properties determine whether a particular solder joint can meet the mechanical, thermal, chemical, and electrical demands placed on it. 

Solder Mlloy Data-Mechanical Properties of Solders and SolderedJoints, I.T.R.I. Pub. No. 656, International Tiu Research Institute. 

Solders. In spite of the wide use and development of solders for millennia, as of the mid-1990s most principal solders are lead- or tin-based alloys to which a small amount of silver, zinc, antimony, bismuth, and indium or a combination thereof are added. The principal criterion for choosing a certain solder is its melting characteristics, ie, soHdus and Hquidus temperatures and the temperature spread or pasty range between them. Other criteria are mechanical properties such as strength and creep resistance, physical properties such as electrical and thermal conductivity, and corrosion resistance. 

Arsenic added ia amounts of 0.1—3% improves the properties of lead-base babbitt alloys used for beatings (see Bearing materials). Arsenic (up to 0.75%), has been added to type metal to increase hardness and castabiUty (21). Addition of arsenic (0.1%) produces a desirable fine-grain effect in electrotype metal without appreciably affecting the hardness or ductihty. Arsenic (0.5—2%) improves the sphericity of lead ammunition. Automotive body solder of the composition 92% Pb, 5.0% Sb, and 2.5% Sn, contains 0.50% arsenic (see Solders and brazing alloys). 

Solders. Modem dental solders are made from mostly corrosion-resistant, nontoxic metals. Minimal quantities of tin and other elements are often added, some of which could produce toxic effects in the unalloyed state. Each solder is used for specific appHcations (180—188) typical compositions and properties of solders used in dentistry are presented in Table 11. Most of the ingredients of solders are resistant to corrosion, and alloying them with other ingredients renders the alloy safe for use in appHances placed in the oral environment. Silver solders corrode, but are used only for temporary appHances. Available products do not contain cadmium, although cadmium was an ingredient of some silver solders up to ca 1980. 

Solders should flow promptly and smoothly over the surfaces of the parts to be joined. This property depends on the surface tension, viscosity, and adhesive properties of the molten solder. Finally, the color of a solder should match that of the metal employed, and its physical properties should be at least as good as those of the metal, in order for the joint not to be a source of weakness (150). 

Automated soldering operations can subject the mol ding to considerable heating, and adequate heat deflection characteristics ate an important property of the plastics that ate used. Flame retardants (qv) also ate often incorporated as additives. When service is to be in a humid environment, it is important that plastics having low moisture absorbance be used. Mol ding precision and dimensional stabiUty, which requites low linear coefficients of thermal expansion and high modulus values, ate key parameters in high density fine-pitch interconnect devices. 

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